1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine. More specifically, the present invention is concerned with a control apparatus for an automotive engine equipped with an exhaust gas circulation apparatus (EGR apparatus) for recirculating an exhaust gas from an engine exhaust system to an engine air intake system.
2. Description of Related Art
Conventionally, various control apparatuses equipped with EGR recirculation for recirculating the exhaust gas from an engine exhaust system to an engine air intake system have been proposed. It should be noted that as this EGR apparatus, there is a so-called "mechanical type EGR apparatus" for controlling the exhaust gas recirculation amount (EGR amount) of the exhaust gas responsively to the negative pressure in the air intake system and the exhaust gas pressure into the EGR valve (exhaust gas recirculation control valve) and the EGR-VM (negative pressure control valve). Also, there are other control apparatuses, i.e., a so-called "electronic control type EGR apparatus" for controlling the EGR amount by the use of an electromagnetically-operated actuator. In general, the former mechanical type EGR apparatus has been well utilized because of the low cost and the high reliability. In other words, in the mechanical EGR apparatus, there is no such a risk that the electric signal line is disconnected, the sensors erroneously sense the operating conditions, and the actuator is erroneously operated due to thermal influences.
The arrangement of the typical mechanical type EGR apparatus will now be summarized. That is, the air intake negative pressure appearing near a throttle valve is introduced into the EGR valve, and this EGR valve opens and closes the EGR path in response to this negative pressure. Both the air intake negative pressure and the exhaust gas pressure are introduced into the EGR-VM. The EGR-VM controls the air intake negative pressure to the EGR valve in accordance with the balance between the air intake negative pressure and the air exhaust pressure. Such EGR apparatus, for instance, has an EGR characteristic shown by a broken line (characteristic curve "La") in FIG. 17. FIG. 17 graphically represents the characteristic of the EGR ratio (EGR amount/air intake amount) with respect to the air intake negative pressure under a preselected engine revolution speed.
However, when the EGR control is carried out by the conventional mechanical type EGR apparatus, there occur the following problems.
That is, since the EGR amount is determined based on the balance in the air intake negative pressure and the exhaust gas pressure in accordance with the above-described conventional mechanical type EGR control, this EGR characteristic cannot be precisely controlled in accordance with the engine operation conditions. As a result, this EGR characteristic results in a low degree of freedom with respect to the optimum characteristic. Since the safety allowance is sufficiently required, this EGR characteristic should be set with having compromise.
As a consequence, when the improvements in the exhaust emission and also fuel efficiency are required due to the specification requirements of the internal combustion engine, such a requirement could not be sufficiently achieved by the conventional mechanical type EGR control. For example, when the emission improvement is required, this characteristic becomes the characteristic curve "Lb" shown by the solid line in FIG. 17, but this requirement could not be satisfied by the conventional EGR control. Also, in such a low load region where combustion becomes unstable, since the air intake negative pressure is high and also the air intake amount is low, accuracy in the EGR control is considerably deteriorated. Thus, this may give adverse influences on the vehicle driveability and the engine knocking characteristic.